This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-198552, filed Jun. 29, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a vertical type power MOSFET having a trenched gate structure.
2. Description of the Related Art
A trench is formed in a semiconductor substrate, and this trench is used to form a trenched gate structure. These trenched gate structures are used in semiconductor devices such as an IGBT (Insulated Gate Bipolar Transistor) and a MOSFET (MOS-type Field Effect Transistor), and this is particularly advantageous for electrical power applications. For example, because a power MOSFET having a trenched gate structure can attain high switching speeds, high current capacities and a breakdown voltage of about several tens volts to 100 volts, they are widely used for switching a power source in portable devices or personal computers.
An n channel type power MOSFET having a trenched gate structure will be described in the following as an example of such a power MOSFET.
It is to be noted that FIG. 20 does not represent a prior art. It is a schematic sectional view of the main portions of an n-channel type trench gate structure power MOSFET which was experimentally designed by the present inventors in the process of achieving the present invention.
That is to say, FIG. 20 illustrates, as an example of a power MOSFET, a cross sectional of one half (half pitch) of a paired vertical type MOSFETS in the plurality of units serially formed on a semiconductor substrate.
The schematic structure is such that an nxe2x88x92 type drift layer 108 and a p type base layer 110 are formed as a laminated body in sequence on the semiconductor substrate used as a drain layer 112, and a trench T is formed on the laminated body. A gate electrode 104 is formed on the surface of the inner wall of the trench T with a gate insulating film 102 interposed therebetween.
The drain electrode 114 is formed at the bottom surface side of the n+ type drain layer 112 which is the semiconductor substrate. An n+ type source region 116 which is adjacent to the gate insulating film 102, and the p+ type region 118 which is formed adjacent thereto are provided on the p+ type base layer 110, and a source electrode 120 is formed so as to extend across these regions 116 and 118.
In this type of power MOSFET, when a predetermined voltage is applied to the gate electrode 104, an inverse layer is formed on the region adjacent to the gate insulating film 102 of the p-type base layer 110, and the power MOSFET turns on and current flows between the source electrode 120 and the drain electrode 114.
However, in the type of power MOSFET shown in FIG. 20, there is a problem that even if the devices are made small, the turn-on resistance or on resistance thereof cannot be effectively reduced.
That is to say, in the case of the type of power MOSFET shown in FIG. 20, the resistance of the device in the on state, that is the on resistance, is determined mainly by the channel resistance component and the drift resistance component. The channel resistance component is the resistance component of the channel region formed on the inverse layer of the pxe2x88x92 type base layer 110 in the on state. On the other hand, the drift resistance component is the resistance component which appears for the on current in the nxe2x88x92 type drift layer 108.
In order to reduce the on resistance of the device, the pitch P of the device unit in FIG. 20 was reduced to thereby increase device density on the semiconductor substrate. That is, the channel density was increased and thus the on resistance of the device was decreased.
Due to the quick advances in semiconductor size reduction processing technology in recent years, the channel density is being rapidly increased, and the channel resistance component is being greatly reduced. Specifically, size reduction has advanced to the extent that the device pitch P is below 0.5 xcexcm. FIG. 20 shows the half pitch structure which is one half the paired device unit. However, in the actual device in which the structure shown in the figure is juxtaposed in both sides thereof, the width of the p-type base layer 110 which is sandwiched between the two adjacent trenched gate structures has been made so small that it is substantially equal to the pitch P, and is less than 0.5 xcexcm.
Further, under these conditions, the on resistance of the recent power MOSFET is such that the above-mentioned drift resistance component has come to account for approximately two-thirds of the total resistance.
That is to say, even when the manufacturing process is further improved and the device pitch P becomes even smaller, there is the problem that significant reduction in the on resistance of the device can not be expected.
For example, in the case of a power MOSFET of the type having a breakdown voltage of 30 volts, it is extremely difficult to reduce the on resistance to 20 mxcexa9mm2 or less.
In order to solve this problem, it is necessary to reduce the thickness t of the drift layer 108, thereby reducing the drift resistance component. In order to do this, a method can be considered in which the gate insulating film 102 is made thicker and when voltage is applied between the gate electrode 104 (the source electrode 120) and the drain electrode 114, the gate insulating film 102 is caused to receive a portion of the applied voltage and thus the thickness of the drift layer 108 can be reduced.
FIG. 21 is a schematic view showing the cross-section structure of the power MOSFET formed based on this concept. The device shown in FIG. 21 is the same as that shown in FIG. 20 except that the thickness of the gate insulating film 102 is greater than that in FIG. 20. Thus the components are indicated by the same reference numerals. That is to say, in the power MOSFET shown in FIG. 21, by making the gate insulating film 102 thicker, the portion by which the thickness is increased receives a portion of the applied voltage, and thus the thickness t of the drift layer 108 is reduced.
However, when the thickness of the gate insulating film 102 is increased in this manner, the threshold voltage of the power MOSFET is increased. As a result, the on resistance is increased by the amount by which the channel resistance is increased when the same gate voltage is applied, and a problem is caused that on resistance of the device can not be effectively reduced.
As described above, in the power MOSFET having this structure, because the on resistance is determined by the drift resistance component, there is the problem that even if the device is made smaller, the on resistance thereof can not be efficiently reduced.
A power MOSFET of one aspect of the present invention comprises: a drain layer having a first conductivity type; a drift layer having the first conductivity type provided on the drain layer; a base layer having a second conductivity type provided on the drift layer; a source region having the first conductivity type provided on the base layer; a gate insulating film formed on the inner wall surface of a trench formed through the base layer and reaching at the drift layer; a gate electrode provided inside the gate insulating film provided in the trench, wherein the gate insulating film has a portion adjacent to the drift layer thicker than the portion adjacent to the base layer, and the drift layer has an impurity concentration gradient higher in the vicinity of the drain layer and lower in the vicinity of the source region along a depth direction of the trench.
According to another aspect of the present invention, a power MOSFET comprises: a drain layer having a first conductivity type; a drift layer having the first conductivity type provided on the drain layer; a base layer having the first conductivity type provided on the drift layer; a source region having the first conductivity type provided on the base layer; a gate insulating film formed on the inner wall surface of a trench formed through the base layer and reaching at the drift layer; a gate electrode provided inside the gate insulating film provided in the trench, wherein the gate insulating film has a portion adjacent to the drift layer thicker than the portion adjacent to the base layer, and the drift layer has an impurity concentration gradient higher in the vicinity of the drain layer and lower in the vicinity of the source region along a depth direction of the trench.